The Java EE 5 Tutorial

Finalizing a Servlet

When a servlet container determines that a servlet should be removed from service
(for example, when a container wants to reclaim memory resources or when it
is being shut down), the container calls the destroy method of the Servlet
interface. In this method, you release any resources the servlet is using and
save any persistent state. The following destroy method releases the database object
created in the init method described in Initializing a Servlet:

public void destroy() {
bookDB = null;
}

All of a servlet’s service methods should be complete when a servlet is
removed. The server tries to ensure this by calling the destroy method only
after all service requests have returned or after a server-specific grace period, whichever
comes first. If your servlet has operations that take a long time to
run (that is, operations that may run longer than the server’s grace period),
the operations could still be running when destroy is called. You must make
sure that any threads still handling client requests complete; the remainder of this
section describes how to do the following:

Keep track of how many threads are currently running the service method.

Provide a clean shutdown by having the destroy method notify long-running threads of the shutdown and wait for them to complete.

Have the long-running methods poll periodically to check for shutdown and, if necessary, stop working, clean up, and return.

Tracking Service Requests

To track service requests, include in your servlet class a field that counts
the number of service methods that are running. The field should have synchronized
access methods to increment, decrement, and return its value.

The service method should increment the service counter each time the method is
entered and should decrement the counter each time the method returns. This is
one of the few times that your HttpServlet subclass should override the service
method. The new method should call super.service to preserve the functionality of the original
service method:

Notifying Methods to Shut Down

To ensure a clean shutdown, your destroy method should not release any
shared resources until all the service requests have completed. One part of doing
this is to check the service counter. Another part is to notify the
long-running methods that it is time to shut down. For this notification, another
field is required. The field should have the usual access methods:

Creating Polite Long-Running Methods

The final step in providing a clean shutdown is to make any long-running
methods behave politely. Methods that might run for a long time should check
the value of the field that notifies them of shutdowns and should interrupt
their work, if necessary.